1. Field of the Invention
The present invention relates generally to communication networks, and in particular to a wireless station of a wireless data network finding at L2 one or more access points that are in communication with a mobility agent, such that the wireless station can associate with such an access point to enable Mobile IP services.
2. Background
Wireless networks such as wireless local area networks (WLANs) are known. In a WLAN, stations are able to communicate wirelessly within a local network. In an infrastructure network, all communication is via an access point that acts as a base station. Any entity on the WLAN can also communicate with a node on any other wired or wireless network that is connected to the WLAN, e.g., via the access point.
Mobile IP is a well known extension of the Internet Protocol (IP) that enables a node on an IP-based network to change its physical attachment point in the network, e.g., the Internet, while retaining the same IP address by which it is identified on its home network. Agents providing Mobile IP services send IP packets that include advertisements of the services they provide. Thus, determining IP services is a process that occurs at L3 of the communication protocol.
While WLANs typically enable a wireless node to move from place to place within a small area, e.g., at a LAN level, without disrupting communications, mobile IP provides IP address mobility across a global network such as the Internet.
A station in a WLAN is not guaranteed to function seamlessly as Mobile IP node. In particular, a prior art wireless station associates with an access point at L2. Because the availability of IP services is determined at L3, prior art wireless stations do not function seamlessly with Mobile IP.
It is desired for a station on a WLAN also to be able to communicate using Mobile IP. In particular, it is desired for the wireless station to determine what IP services, if any, are available to access points prior to attaching to an access point.
The following discussion provides a brief overview of Mobile IP and wireless LANs, including definitions. A review of the need for the invention follows the discussion.
Mobile IP
The purpose of Mobile IP is to enable a mobile node to retain its home IP address even when it is away from its home subnetwork of an overall network such as the Internet. This is accomplished by the addition of a mobility agent, a router known in MIPv4 as a foreign agent, that provides the node with a “care-of” address for the mobile node. MIPv6 provides a slightly different mechanism. The care-of address is part of the IP packet itself, so that foreign agents are not required. The mobile node's home network similarly adds a mobility agent, in this case a router known as a home agent, which forwards IP packets addresses to the mobile node's IP address to the node's care-of address on the foreign network.
When a MIPv4 mobile node determines that it is not attached to its home network, it listens for router advertisements from a foreign agent. Such foreign agent advertisements contain one or more care-of addresses. After obtaining a care-of address, the mobile node then registers with the foreign agent, and subsequently, registers the care-of address with its home agent. Once these registrations are complete, the home agent intercepts IP packets containing the IP address of the mobile node and forwards them, via an IP tunnel, to the care-of address. When these IP packets reach the foreign network, the foreign agent redirects them to the mobile IP node at its care-of address. Thus the foreign agent removes them from the tunnel and the home agent together act as proxies for the mobile node's IP address, routing inbound and outbound traffic to and from the mobile node using its home IP address. When a mobile note again changes its attachment point to the Internet, it locates a new foreign agent, obtains a new care-of address, and re-registers with its home agent.
Note that the providing of Mobile IP services by foreign agents and home agents uses IP packets, and thus occurs at the network layer (layer-3).
According to RFC 3344, there is only one active care-of address for each foreign agent or access router, and therefore a single reverse tunnel to the Home Agent needs to be set up.
Wireless LANs
WLANs conform to one or more known protocols, such as the various versions of the IEEE 802.11 standard. An infrastructure WLAN includes an access point (AP) to act as a base station in the WLAN so that any wireless communication between stations in the WLAN occurs via the AP. Typically, the AP is connected to a wired network such as a LAN which in turn may be connected to a wide area network (WAN) such as the Internet. Any station in the WLAN can thus communicate with both the wired LAN or WAN via the AP.
According to the IEEE 802.11 standards, a station attempts to associate with an AP by scanning in “RF monitor mode” for signals, known as “beacon signals,” that periodically originate from an AP. Beacon signals, in addition to other functions, serve to uniquely identify the AP within the WLAN. When a station detects a beacon, it switches into send-receive mode and executes an association protocol with the AP. At the successful end of the association protocol, the station is said to be associated with the AP.
Once a station associates with an AP, the AP is responsible for delivering network traffic to the station (and other associated stations). While an AP may associate with multiple stations, a station may associate with only one AP at a time, even though there may be more than one AP that the station can physically communicate with. When an associated station loses communication with its AP, the station must re-associate, either with a different AP, or the same AP.
The association between a station and an AP is initiated by a station, not an AP. After receiving one or more beacon signals, the station selects an AP it can communicate with, e.g., after receiving a beacon signal, and begins the association protocol. The criteria for a station selecting one AP over another is not specified in the IEEE 802.11 standard, but in practice, at minimum it is based on the quality of the wireless signal from the available APs. As previously noted, once associated, the station does not directly wirelessly communicate with any station not associated with its AP.
Note that the associating of a station with an AP occurs at the link layer (layer-2) of the protocol. In contrast, the providing of Mobile IP services by foreign agents and home agents uses IP packets, and thus occurs at the network layer (layer-3). Because the association procedure occurs at a lower protocol layer than Mobile IP services, the association between an AP and a station occurs at a protocol layer wherein there is ignorance of Mobile IP services, and thus occurs without regard to whether the AP is in communication with any mobility agents.
Therefore, there is a need in the art for a method and apparatus that enables a station of a WLAN to discover, prior to association, whether one or more candidate APs for association that are in communication with Mobile IP agents, e.g. with home agents and/or foreign agents. There also is a need subsequently to select for association one of the APs in communication with a mobility agent so that the wireless station can benefit from and/or provide Mobile IP services.
Definitions
Some of the following definitions are applicable to a wireless local area network (WLAN) that conforms to one of the IEEE 802.11 standards. Other WLAN standards exist and future ones may be developed. The present invention is not restricted to working with IEEE 802.11 conforming networks.
Access Point (AP): In the IEEE 802.11 standards, an AP is a wireless base station through which communication is managed in an infrastructure network (see BSS). An AP may be in communication with other wired or wireless networks. When compared to functions in a wired network, it is analogous to an Ethernet hub with optional access control to other networks.
Access Router: A router that provides access between two or more networks, for example, between a WLAN and a LAN, or between a LAN and the Internet. See also Mobility Agent.
Ad Hoc Network: See IBBS.
Association: In the IEEE 802.11 standards, association is the procedure used to establish a binding between an access point and a station, to make the station part of the wireless network managed by the AP. Note that prior to association, a station may be in passive scanning mode, called RF monitor mode herein (see below), but while associated the station is in send-receive mode (see below), called “station” mode in the case of an IEEE 802.11 STA.
Associated station: In an IEEE 802.11 BSS (or ESS) WLAN, communications between stations and other networks is managed by an AP. A station becomes an associated station when it has been successfully accepted by the AP to be part of the wireless network managed by that AP.
Beacon: A periodic 802.11 frame transmitted by an AP that provides information required by stations for establishing and maintaining an association with the AP. The beacon contains the unique identifier called the Service Set Identifier (SSID) for the wireless network of the AP. See also SSID.
BSS: In the IEEE 802.11 standards, a Basic Service Set (BSS) is a single infrastructure wireless network in which all wireless communication occurs via a single access point. See also below under IBSS: an independent BSS.
ESS: An IEEE 802.11 Extended Service Set is a WLAN that includes two or more BSSs, the APs of which may provide service to overlapping physical areas. The APs behave as a single logical router for all nodes in the ESS.
Foreign Agent (FA): A type of Mobility Agent, specifically an IPv4 router on a mobile node's visited network that provides routing services to the mobile node while registered. The foreign agent detunnels and delivers IP packets that were tunneled by the mobile node's home agent to the mobile node. For IP packets sent by a mobile node, the foreign agent may serve as a default router for registered mobile nodes. Foreign agents do not exist in Mobile IPV6. The IP packets themselves include the care-of address and the services provided by a foreign agent in IPv4 is provided by an access router.
Foreign Agent Advertisement: A Mobility Agent advertisement sent by a foreign agent. See Mobility Agent advertisement.
Foreign Network: Any network other than the mobile node's Home Network.
Home Agent (HA): A type of mobility agent, specifically a Mobile IPv4 or Mobile IP version 6 router on a mobile node's home link that maintains current location information for the mobile node in the form of a current care-of address and, while the mobile node is away from home, intercepts packets on the home link destined to the mobile node's home address and tunnels packets for delivery to the mobile node's registered care-of address. A Home Agent is a mobility agent.
Home Address: In Mobile IP, an IP address that is assigned for an extended period of time to a mobile node. It remains unchanged, regardless of where the node is attached to the network.
Home Network: A network, possibly virtual, having a network prefix matching that of a mobile node's home address. Note that standard IP routing mechanisms will deliver IP packets destined to a mobile node's home address to the mobile node's home network.
IBSS: In the IEEE 802.11 standards, an Independent Basic Service Set (IBSS) is a WLAN with two or more wireless nodes that communicate directly without associating with an AP. Also referred to as an ad hoc wireless network in contrast to an infrastructure wireless network.
Infrastructure Network: A wireless network in which all STAs communicate via an AP. See BSS and ESS.
IETF: Internet Engineering Task Force, the standardization body for Internet related technology.
IP: the Internet Protocol, e.g., IP version 4 (IPv4) or IP version 6 (IPv6).
Interface: An interface is used herein in several contexts. In the context of the network layer, e.g., IP including mobile IP, an interface (also an L3 interface) is an L3 node's attachment to a link at L2. In this definition, a node is a device that implements IP, and an L2 link is a communication facility or medium over which nodes can communicate at the link layer (L2), such as a simple or bridged Ethernet. For example, a wireless network device provides a bi-directional interface to a WLAN after a station associates with the AP of the WLAN.
Interface Identifier: A number used to identify a node's interface on a link. In one embodiment, the interface identifier is the set of remaining low-order bits in the node's IP address after the subnet prefix.
Internet Router Discovery Protocol (IRDP): A protocol for routers on a network broadcast their presence to other nodes on the network. A router supporting this protocol periodically broadcasts an Internet Control Message Protocol (ICMP) IP packet containing a “router advertisement.” A node on the network may at any time invoke a router advertisement by broadcasting a “router solicitation.” Mobile IP defines (RFC 3344) extensions to this protocol that identifies the router advertisement as a “home agent advertisement” and/or a “foreign agent advertisement.” See Mobility Agent Advertisement below.
Mobility Agent (MA): In MIPv4, the Mobility Agents are foreign agents that receive data from home agents, and the home agents. In IPv6, the Mobility Agents are access routers and home agents. One aspect of the present invention is the discovery at L2 of mobility agents such as home agents, foreign agents and/or access routers. The term Mobility Agent is used herein as a general term encompassing foreign agents, home agents, and access routers.
Mobility Agent Advertisement: Mobility agent advertisements are transmitted by a mobility agent to advertise its services. Mobile nodes use these advertisements to determine their current point of attachment e.g., to the Internet. A mobility agent advertisement is an Internet Control Message Protocol (ICMP) IP packet that includes a Router Advertisement that has been extended to also carry a mobility agent advertisement Extension. The exact function of the advertisement—home agent, foreign agent, or access router—is identified by a FLAGS field in the extension header.
Mobile IP, IP Mobility: An IETF document (RFC 3344) that describes the infrastructure in which a mobile node may change its attachment point from one network or subnetwork to another without changing its home (IP) address.
Mobile IPv4: Mobile IP for IP version 4. Same as (RFC 3344).
Mobile IPv6: Mobile IP to work with IP version 6. See, for example, David B. Johnson, Charles E. Perkins, and Jari Arkko: “Mobility Support in IPv6” (document title “draft-ietf-mobileip-ipv6-18.txt”, available on the Internet at http://www.ietf.org/internet-drafts/draft-ietf-mobileip-ipv6-18.txt. In Mobile IP for IP version 6, the care-of-address is included in the IP packet itself. Therefore, in Mobile IPv6, foreign agents do not exist. The only Mobile IP requirement placed upon the foreign network is to provide an access router capable of routing packets to the Mobile IP node's home address.
Mobile Network: A network that moves together with a Mobile Router. See also Mobile Router.
Mobile Node: In Mobile IP, a node in a network, e.g., a host or router that changes its point of attachment from one network or subnetwork to another. A mobile node may change its location without changing its IP address; it may continue to communicate with other Internet nodes at any location using its (constant) IP address, assuming link-layer connectivity to a point of attachment is available. Mobile nodes may remain at a stationary location or be in motion. For purposes of the present invention, mobile stations and portable stations as defined in IEEE 802.11 can also be mobile nodes if they support Mobile IP.
Mobile Router: A mobile node that is a router. A mobile router provides for the mobility of one or more networks moving together, for example, on an airplane or a ship. The nodes connected to a network served by the Mobile Router may themselves be mobile nodes or routers. Using the present invention, a STA of an infrastructure wireless network that provides routing may be a mobile router.
Mobile Station: In the IEEE 802.11, a type of station that is able to use network communications while in motion. A mobile station is a type of mobile node when Mobile IP enabled. See also portable station.
Portable Station: In the IEEE 802.11 standards, a type of station that may be moved from location to location, but that is only able to use network communications while stationary at a fixed location (i.e., not in motion). A portable station is a type of mobile node when Mobile IP enabled. See also mobile station.
Re-association: In the IEEE 802.11 standards, The process whereby a station associated with an AP transfers association to another—possibly the same—AP. Also commonly referred to as a “hand-off.”
RF Monitor Mode: As used herein, a mode for a station during which a wireless network device listens to all MAC frame types (control, management and data) regardless of the AP from where the frames originate or belong to. This means that, if the radio module is in an area covered by more than one AP, it will listen to the traffic broadcast in each AP or wireless network. The purpose of this mode is essentially to enable a wireless station to “scan” for broadcasts from APs, e.g., the broadcast of beacons in the IEEE 802.11 standard, in order to locate an AP with a good enough signal strength with which to associate. It is assumed that one or more of the wireless network device or devices used in embodiments of the invention have an RF monitor mode or equivalent. Note that the process of listening for-beacons is called “passive scanning” in the IEEE 802.11 standard. IEEE 802.11 beacons contain information about the access point, including the service set identifier (SSID) of the WLAN of the AP, supported data rates, and so forth. In a normal IEEE 802.11 scan, the wireless network device provides this information along with the signal strength to compare access points and decide upon which one to associate with. A wireless network device in RF monitor mode can receive from more than one AP, but not transmit information. Other modes include send-receive mode, also called station mode.
Router: A router is a device implementing IP that forwards IP packets not explicitly addressed to itself.
Router Advertisement: An IP packet, in particular an ICMP IP packet periodically broadcasts to all nodes on the network served by a router. In the Mobile IP support provided by IPv4, extensions to the advertisements, referred to as to Agent Advertisements, carry information identifying the router as a foreign agent and/or a home agent.
Send-Receive Mode: In the IEEE 802.11 standard, the communication state wherein a station is able to communicate bidirectionally with its associated AP. Also called station mode. See also RF Monitor mode.
SSID: In the IEEE 802.11 standards, a Service Set Identifier of a wireless local area network. In a simple infrastructure network (BSS), the SSID also identifies the AP. The SSID is sometimes called the wireless network identifier. Many wireless network devices for the IEEE 802.11 standard give a special meaning to a SSID with zero length, called the broadcast SSID. If requested to associate with the broadcast SSID, the station will go to RF monitor mode and scan for all the 802.11 broadcast beacons containing an SSID. In the prior art, the station may then associate with the first AP it finds independent of any L3 information such as whether or not the AP is in communication with a mobility agent.
Station (STA): A node of a wireless network conforming to one of the IEEE 802.11 standards, in particular, a device that contains an IEEE 802.11 conformant medium access control (MAC) and physical layer (PHY) interface to the wireless medium. In the context of the present invention, a station includes a wireless network device that, after associating with an access point of a wireless network, provides a bi-directional interface to that wireless network.
Visited Network: In Mobile IP, a network other than a mobile node's home network, to which the mobile node is currently attached. When attached to a foreign network, the node is said to be “visiting” that network.
Wired Equivalent Privacy (WEP): An 802.11 security protocol for wireless networks. The WEP encryption method is designed to provide security.
Wireless Network device: A device that includes a wireless transceiver, i.e., a wireless receiver and a wireless transmitter, and capable of bi-directional wireless communication. For purposes of the invention, a wireless network device can operate in RF Monitor mode prior to the station containing the wireless network device associating with a wireless network. A wireless network device after associating with an access point of a wireless network, provides a bi-directional interface to that wireless network.
WLAN: Wireless Local Area Network, e.g. a wireless network conforming to one of the IEEE 802.11 standards.
Note that Mobile IP and 802.11 use different terminology and nomenclature for similar network components. For example, Mobile IP refers to a Mobile IP-enabled network node as a “Mobile Node.” IEEE 802.11 defines two types of stations: a “mobile” station, able to communicate while in motion, and a “portable” station able to communicate only while at a fixed location. The term Mobile Node when referring to a wireless station means a wireless station that is Mobile IP enabled and that is either an IEEE 802.11 “mobile station” or an IEEE 802.11 “portable station” in the case the station conforms to one of the IEEE 802.11 standards.